1. Field of the Invention
This invention relates to a ceramic honeycomb structural body used for a catalyst purifying harmful components such as hydrocarbon, carbon monoxide and nitrogen oxides included in an exhaust gas discharged from an internal combustion engine of automobile or the like, or a catalyst purifying various harmful components discharged from a boiler, and more particularly to a ceramic honeycomb structural body having a plurality of open-ended cells defined by thin cell walls.
2. Description of Related Art
The ceramic honeycomb structural body having a plurality of open-ended cells defined by a cylindrical hollow-shaped outer wall and many cell walls is used from the old time as a support for the catalyst purifying harmful components such as hydrocarbon, carbon monoxide and nitrogen oxides included in an exhaust gas discharged from an internal combustion engine of automobile or the like, or the catalyst purifying various harmful components discharged from a boiler. The development of such a ceramic honeycomb structural body traces the history of thinning the thickness of the cell wall from early time of development up to now.
As the thickness of the cell wall becomes thinner, the mechanical strength of the ceramic honeycomb structural body lowers, so that there are proposed various techniques for solving this problem. In general, it can be considered that if the mechanical strength is born only by the outer peripheral wall of the ceramic honeycomb structural body, the cell walls inside the structural body can freely be thinned. However, the materials constituting the ceramic honeycomb structural body are actually limited from a viewpoint of heat resistance and thermal shock resistance, and also the thickness of the outer peripheral wall is limited to not less than about 1.5 mm from restriction in the production, so that the mechanical strength required for the ceramic honeycomb structural body could not be obtained only by the outer peripheral wall.
For this end, JP-B-62-18797 and JP-B-61-60320 disclose a technique wherein the thickness of all cell walls existing in the outer peripheral portion of the ceramic honeycomb structural body is thickened and the thickness of the cell wall inside the structural body is thinned for preventing the lowering of the mechanical strength. According to this technique, however, since the thickness of the cell wall existing in the outer peripheral portion of the ceramic honeycomb structural body is thickened, the mechanical strength can be developed, but there is caused a problem that it becomes difficult to flow the exhaust gas in open-ended cells in the vicinity of the thick cell wall in the outer peripheral portion.
Further, JP-B-5-41296 discloses a technique wherein a thickness of a cell wall contacting with the outer peripheral wall of the ceramic honeycomb structural body having a rectangular shape at lateral section is gradually increased toward the outer peripheral wall. This technique is a technique applicable only to an extremely limited condition or a condition that the outer peripheral wall of the ceramic honeycomb structural body having a rectangular shape is formed by slits in an extrusion die and hence a dimensional tolerance of the outer peripheral shape is very loose. Therefore, there is caused a problem that the above technique cannot be applied to the catalyst for the automobile as it is.